Strategy of thermodynamic and kinetic improvements for Mg hydride nanostructured by immiscible transition metals

Abstract High hydrogen density and low material costs make Mg as one of the most promising candidates for solid-state hydrogen storage. However, the practical applications of Mg are restricted by high reaction temperature and slow kinetics of hydrogen absorption/desorption. Here we present the improvements of both thermodynamics and kinetics of the hydride/deuteride of Mg (MgH2/MgD2) by utilizing the immiscible Mg–Cr system. Nanometer-sized MgD2 domains with the average crystallite size of ~10 nm embedded in a Cr matrix are formed in deuterated Mg0.25Cr0.75. X-ray diffraction and nuclear magnetic resonance spectroscopy studies show that the MgD2 domains are heavily distorted, which leads to the thermodynamic destabilization lowering the reaction temperature. Mg0.25Cr0.75 can reversibly absorb and desorb hydrogen/deuterium at a low temperature of 473 K. The enthalpy ΔH for deuterium desorption of Mg0.25Cr0.75−D is 72.1 kJ mol−1−D2, which is lower than ~74 kJ mol−1−D2 for bulk MgD2. The apparent activation energy for hydrogen desorption of Mg0.25Cr0.75−H is decreased to 75 kJ mol−1 from ~160 kJ mol−1 for bulk Mg, in which the dehydrogenation of nanometer-sized MgH2 is controlled by one-dimensional diffusion of hydrogen. Our work demonstrates that MgH2 nanostructured by an immiscible matrix is a useful strategy to alter the thermodynamic and kinetic properties.